1. Field of the Invention
This disclosure pertains to packaging for micromachined fluid sensors, and more specifically to such a package especially suitable for mounting where there is limited vertical clearance.
2. Description of the Prior Art
Micromachined (fabricated by integrated circuit-type technology) fluid pressure sensors are well known. A typical such sensor is housed in a plastic dual in-line package (DIP) of the type commonly used for integrated circuits. Such packages are adapted for the sensor application by providing one or two holes (ports) allowing the fluid access to the sensor itself. Such sensors typically require fluid to be able to contact both the upper side and the lower side of the (silicon) substrate in which the sensor is formed in order to measure pressure. In e.g. gage applications one of the access ports merely provides exposure to the ambient atmosphere.
Typically, at least the port in the upper portion of the package lid (cover) is in communication with an extension which is a nozzle (tube) approximately 1 cm long defining a central bore and thereby communicating via the port with the package interior. This nozzle is suitable for attachment to a suitable length of tubing for conducting fluid, the pressure which is to be measured, to the upper surface of the sensor inside the package. Typically a second port in the bottom surface of the body of the package allows a second fluid (such as the ambient atmosphere) to contact the bottom surface of the sensor.
The second port in some applications (where the second fluid is not the ambient atmosphere) is also attached to a similar second nozzle (tube) extending from the package body bottom surface, again this nozzle defining a central bore in communication with the interior of the package for access to the lower surface of the sensor. Due to the length of the package leads which electrically connect the sensor to the supporting printed circuit board and also fasten it thereto mechanically, a hole must be drilled through the printed circuit board to accommodate this second nozzle. Any tubing to be attached to the second nozzle is then attached on the undersurface of the printed circuit board.
This configuration has several drawbacks. One is that the nozzle extending vertically out of the package lid requires a significant amount of vertical clearance in order to attach the tubing to it, and the end of the tubing must bend at 90.degree. at the point of connection to the end of the nozzle. This undesirably requires considerable vertical clearance to whatever is located above the sensor package, e.g. another printed circuit board or some other structure inside an instrument.
Similarly, the need to drill a hole through the printed circuit board to accommodate the second nozzle complicates assembly, as does the need to locate any tubing connecting to the second nozzle on the undersurface of the printed circuit board.
Another drawback in the assembly process is that the nozzle or port in the bottom surface of the package must be temporarily capped or sealed during assembly onto the printed circuit board to prevent contaminants from entering therein, especially during the chemical cleaning of the printed circuit board. However, this sealing is very difficult, especially for the case when the package does not have a nozzle on its lower port but merely has the port to admit ambient air; it is very difficult to seal this port since the port is almost in contact with the printed circuit board. Therefore access is difficult if not impossible once the package has been mounted on the circuit board but during the necessary circuit board cleaning.
It has been found by the present inventors that the attachment of the tubes to both the lid and body of the package, such tubes requiring a 90.degree. bend, creates a certain amount of internal strain in the package and stress on the tubes, due to the 90.degree. bend and the need to bend and attach the bent tubing to the nozzles. This has been found to induce stress on the sensor die (substrate) due to the accompanying stresses on the package. This undesirably tends to cause a certain amount of failure of the package and/or the sensor die.